A well known use of radiation image storage phosphors is in the production of X-ray images. In U.S. Pat. No. 3,859,527 a method for producing X-ray images with a photostimulable phosphor, which are incorporated in a panel is disclosed. The panel is exposed to incident pattern-wise modulated X-ray beam and as a result thereof the phosphor temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are processed in order to produce a visible image. For this purpose, the phosphor should store incident X-ray energy in an amount as much as possible and emit stored energy in a negligible amount before being stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
Since in the above described X-ray recording systems the X-ray conversion screens are repeatedly used, it is important to provide them with an adequate topcoat for protecting the phosphor containing layer from mechanical and chemical damage. This is particularly important for photostimulable radiographic screens where screens are often transported in a scanning module—wherein the stimulation of the stored energy takes place—while not being not encased in a cassette but used and handled as such, without protective encasing. A protective layer can be coated onto the phosphor containing layer by directly applying thereto a coating solution containing a film-forming organic solvent-soluble polymer such as nitrocellulose, ethylcellulose or cellulose acetate or poly(meth)acrylic resin and removing the solvent by evaporation. According to another technique a clear, thin, tough, flexible, dimensionally stable polyamide film is bound to the phosphor layer as described in EP-A 0 392 474. According to still another well-known technique a protective overcoat is produced with a radiation-curable composition, e.g. by making use of a radiation curable coating as protective top layer in a X-ray conversion screen as described in EP-A 0 209 358, in JP-A 61/176900 and in U.S. Pat. No. 4,893,021. So in U.S. Pat. No. 6,120,902 an intensifying screen is disclosed carrying a radiation cured protective layer and having a determined unevenness. U.S. Pat. No. 4,059,768 further illustrates use of polymeric beads containing fluoro-moieties in intensifying screens in order to have screens with good transportability. In U.S. Pat. No. 5,401,971 storage phosphor screens are disclosed comprising a protective layer coated from a solution, in butanone, of a miscible blend of poly(vinylidene fluoride-co-tetrafluoroethylene) and poly(1 to 2 carbonalkyl)-methacrylate.
More recently as more particular screens yielding X-ray images with good quality should have an increased physical strength in order to be transported in a scanner without risk of jamming, further withstanding wear and tear of transporting, with no or low risk for electrical charging, provisions have been taken as set forth in EP-Application No. 01 000 694, filed Dec. 3, 2001, wherein said protective layer is radiation cured.
In order to provide moistureproof use and processing of phosphor screens or panels suitable for use in radiation detectors, said detectors are advantageously overcoated with a moistureproof protective layer, provided thereupon by chemical vapor deposition, just as needle-shaped crystals have advantageously been deposited. More preferably said radiation detector is a phosphor screen, i.e. an intensifying screen or a storage phosphor screen or panel, wherein, in a further preferred embodiment, said phosphor screen is a needle storage phosphor screen, and wherein said moistureproof protective layer is a “Parylene” (poly-p-xylylene) layer as disclosed in EP-A's 1 286 362, 1 286 363, 1 286 364 and 1 286 365.
As a further development described e.g. in U.S. Pat. No. 3,859,527 an X-ray recording system is disclosed wherein photostimulable storage phosphors are used that, in addition to their immediate light emission (prompt emission) on X-ray irradiation, have the property to store temporarily a large part of the energy of the X-ray image which energy is set free by photostimulation in the form of light different in wavelength characteristic from the light used in the photostimulation. In said X-ray recording system the light emitted on photostimulation is detected photo-electronically and transformed in sequential electrical signals. Basic constituents of such X-ray or radiographic imaging system operating with storage phosphors are an imaging sensor containing said phosphor, normally a plate or panel, which temporarily stores the X-ray energy pattern, a scanning laser beam for photostimulation, a photo-electronic light detector providing analog signals that are converted subsequently into digital time-series signals, normally a digital image processor which manipulates the image digitally, a signal recorder, e.g. magnetic disk or tape, and an image recorder for modulated light-exposure of a photographic film or an electronic signal display unit, e.g. a cathode ray tube.
A survey of lasers useful in the read-out of photostimulable latent fluorescent images is given in the periodical Research Disclosure Volume 308 No. 117 p. 991, 1989.
From the preceding description it is clear that said X-ray plates or panels only serve as intermediate imaging elements and do not form the final record. The final image is made or reproduced on a separate recording medium or display. The phosphor plates or sheets can be repeatedly re-used, provided that before re-use of the photostimulable phosphor panels or sheets a residual energy pattern is erased by flooding with light.
Besides stringent mechanical requirements set out hereinbefore image quality of the image storage panels, exceptionally severe demands should be fulfilled, especially with respect to image definition, i.e., sharpness. The sharpness does not depend upon the degree of spread of the light emitted by the stimulable phosphor in the panel, but depends on the degree of spread of the stimulable rays in the panel: in order to reduce this spread of light a mixture can be made of coarser and finer batches in order to fill the gaps between the coated coarser phosphor particles. A better bulk factor may be attained by making such a mixture, resulting in a loss in sensitivity unless the phosphor grains are only slightly different in sensitivity. For intensifying screens this topic has already be treated much earlier by Kali-Chemie and has been patented in U.S. Pat. Nos. 2,129,295; 2,129,296 and 2,144,040. Otherwise reduction of graininess is also strived at and can be realized, e.g. by providing a layer arrangement wherein the protective layer is unsharply depicted as disclosed in DE-A 3031267. Phosphor layer thickness may further give rise to increased unsharpness of the emitted light. This is more unfavourable when the weight ratio between the amount of phosphor particles and the amount of binder decreases for the same coating amount of said phosphor particles. Enhancing the weight ratio amount of phosphor to binder in order to provide sharper images, by decreasing the amount of binder, leads to unacceptable manipulation characteristics of the screen due to e.g. insufficient elasticity and brittleness of the coated phosphor layer in the screen. One way in order to get thinner coated phosphor layers without changing the coated amounts of pigment and of binder makes use of a method of compressing the coated layer containing both, binder and phosphor, at a temperature not lower than the softening point or melting point of the thermoplastic elastomer as has been described in EP-A 0 393 662. Another way free from compression manufacturing techniques has been proposed in PCT-filing WO 94/0531, wherein the binding medium comprises one or more rubbery and/or elastomeric polymers providing improved elasticity of the screen, high protection against mechanical damage, high ease of manipulation, high pigment to binder ratio and an improved image quality, especially sharpness.
Radiographs obtained from the well-known intensifying screen generation showing improved visualisation therefore comprise, more particularly in favour of sharpness, a blue-light absorbing, complementary yellow dye, as described e.g. in EP-A 0 028 521. Early references referring to the improvement of sharpness of radiation image storage panels, related with the addition of a colorant to the panels, as in U.S. Pat. No. 4,394,581, take into account that a dye or colorant is added to the panel so that the mean reflectance of said panel in the wavelength region of the stimulating rays for said stimulating phosphor is lower than the mean reflectance of said panel in the wavelength region of the light emitted by said photostimulable phosphor upon stimulation thereof. In U.S. Pat. No. 4,491,736 more specifically an organic colorant is disclosed which does not exhibit light emission of longer wavelength than that of the stimulating rays when exposed thereto. EP-A 0 165 340 and the corresponding U.S. Pat. No. 4,675,271 disclose a storage phosphor screen showing a better image definition by incorporation of a dye. An analoguous effect brought about in phosphor layers of image storage panels by incorporation of dyes or colorants has further been described in EP-A 0 253 348 and the corresponding U.S. Pat. No. 4,879,202 and in EP-A 0 288 038. In favour of sharpness of radiation image storage panels colored with a colorant so that the mean reflectance of said panel in the wavelength region of the stimulating rays for the photostimulating phosphor present in its storage phoshor layer is lower than the mean reflectance of said panel in the wavelength region of the light emitted by said photostimulable phosphor upon stimulation thereof, a triarylmethane dye having at least one aqueous alkaline soluble group was selected, wherein said dye should be present in at least one of said support, said phosphor layer or an intermediate layer between said support and said phosphor layer as disclosed in EP-A 0 866 469 and the corresponding U.S. Pat. No. 5,905,014. In case wherein an antihalation undercoat layer is present in the panel between the phosphor layer and a support having reflective properties, a layer arrangement of intermediate layers is preferred in order to get an optimized relationship between mechanical characteristics, speed and sharpness: an intermediate layer arrangement between (subbed) support and phosphor layer substantially consisting of an antihalation undercoat layer containing one or more dye(s), wherein said layer is situated more close to said support, and an adhesion improving layer situated more close to the said layer of storage phosphor particles, and wherein said adhesion improving layer is hardened to a lesser extent than said antihalation undercoat layer, is highly recommended as has been illustrated in EP-Application No. 02 100 195, filed Feb. 28, 2002.
In order to obtain a reasonable signal-to-noise ratio (S/N) the stimulation light should be prevented from being detected together with the fluorescent light emitted on photostimulation of the storage phosphor which should reach the detector. Therefore a suitable filter means should be used preventing the stimulation light from entering the detecting means, e.g. a photomultiplier tube. Because the intensity ratio of the stimulation light is markedly higher than that of the stimulated emission light, i.e. differing in intensity in the range of 104:1 to 106:1 (see published EP-A 0 007 105, col. 5) a very selective optical filter should be mounted between the phosphor plate and the detector. Suitable filter means or combinations of filters may be selected from the group of cut-off filters, transmission bandpass filters and band-reject filters. A survey of filter types and spectral transmittance classification is given in SPSE Handbook of Photographic Science and Engineering, Edited by Woodlief Thomas, Jr.—A Wiley-Interscience Publication—John Wiley & Sons, New York (1973), p. 264–326. In praxis a useful filter should absorb red laser light (generated by a laser source, used as stimulating light source) and be transparent for blue light, emitted upon stimulation. Most preferred filters are made of colored glass, but a disadvantage is its thickness. Even after optimisation a filter composed of dielectric layers has a minimum thickness of 2.5 mm at least. Most preferably the thickness of the optical filter should be minimized in order to increase collection efficiency by the detector: a higher collection efficiency indeed enhances the number of detected quanta, thereby improving the signal-to-noise ratio (SNR).
Otherwise the collection efficiency highly depends on the distance between storage phosphor screen or panel and detector. A smaller distance provides a higher collection efficiency. Optical means providing high collection efficiency, such as a Fiber Optic Plate or “FOP” can only be applied in combination with a very thin filter (as long as no very thin filter was available those techniques could e.g. not be applied).
In the past decades quite a lot of research has been performed in order to produce very thin optical filters, based on use of suitable dyes or colorants, absorbing radiation in the desired wavelength range. Burden has until now frequently been laid upon photochemical stability of such dyes. Therefor it remains an ever lasting demand to further direct investigations related with improvement of sharpness.
From a point of view of phosphor morphology or structuration in a phosphor layer, more particularly related with measures improving sharpness, needle-shaped phosphors present in binderless layers are particularly suitable for use in flat scanner plates. For screens having been coated from hydrophilic coating solutions, there is no problem when further measures are taken in favour of sharpness, related with respect to the selection of a dye, provided that, besides suitable radiation absorption properties, the dye has a hydrophilic moiety in its structure, and that a perfect resistance to degradation under the influence of radiation and bleaching phenomena as well is guaranteed. More questionable however is the selection of dyes with hydrophobic moieties, more particularly when hydrophilic moieties should be avoided as coating should be performed in non-aqueous coating solutions, preferred as in cases wherein phosphor particles are particularly sensitive to moisture.